Document: ARN-3-34-29

Elevated carbon dioxide inhibits nitrate photoassimilation.

BLOOM, A.J.* and D.R.SMART

University of California, Davis, CA 95616 ¹

Abstract:
We recently confirmed in short-term leaf gas exchange measurements and longer-term plant growth experiments that elevated CO₂ inhibits nitrate photoassimilation. Shoot assimilatory quotient (AQ) -- the ratio of CO₂ assimilated to O₂ evolved during photosynthesis -- serves as a non-destructive measure of shoot photoassimilation. In wheat plants grown at 360 ppm CO₂ and measured at lower CO₂ concentrations, the AQ decreased markedly when the nitrogen source was shifted from ammonium to nitrate, indicating that the leaves were photoassimilating nitrate. Wheat grown at 360 ppm or 700 ppm CO₂ and measured at higher CO₂ concentrations, exhibited little change in AQ under the two nitrogen sources, indicating diminished nitrate photoassimilation. Wheat plants, grown for two weeks in environmental chambers controlled at 360 or 700 ppm CO₂, received nutrient solutions containing either nitrate or ammonium as the sole nitrogen source. Nitrogen source had little effect on root parameters or on shoot growth and leaf organic N content in the plants grown at 360 ppm. In the plants grown at 700 ppm, however, shoot growth and leaf organic N content were dramatically less under nitrate nutrition. Were CO₂ inhibition of nitrate photoassimilation common among plants, it would offer a mechanistic explanation for several other unresolved phenomenon including (a) plant acclimation to CO₂, (b) CO₂ inhibition of respiration, (c) the localization of nitrate assimilation in the mesophyll of C₄ species, and (d) the central role of nitrogen availability in determining ecosystem response to CO₂ enrichment. Moreover, it suggests that rising CO₂ levels will have yet unforeseen effects upon nitrogen cycling in natural and managed ecosystems because unassimilated nitrate may leach into groundwater or increase nitrogen trace gas emissions.

Keywords: elevated CO2, nitrate, ammonium, photoassimilation

This abstract is being presented at: 2:45 PM in session:
Oral Session #30: Effects of Elevated Carbon Dioxide.